4. 4
Definition prodrug:
Prodrugs can be defined as pharmacologically inert chemical
derivatives that can be converted in vivo to the active drug
molecules, enzymatically or non-enzymatically, to exert a
therapeutic effect.
In simplified terms, prodrugs are masked forms of active
drugs that are designed to be activated after an enzymatic or
chemical reaction once they have been administered into the
body.
Prodrugs are considered to be inactive or at least significantly
less active than the released drugs.
7. 7
CONCEPT OF PRODRUG
Many therapeutic drugs have undesirable properties that may
become pharmacological and pharmaceutical barriers in clinical drug
application.
Among the various approaches to minimize the undesirable drug
properties, while retaining the desirable therapeutic activity, the
chemical approach using drug derivatization offers the highest
flexibility of improving drug efficacy.
The prodrug approach can be useful in the optimization of the clinical
application of a drug.
Numerous prodrugs have been designed and developed to overcome
pharmacological and pharmaceutical barriers in clinical drug
application, such as low oral drug absorption, lack of site specificity,
chemical instability, toxicity, and poor patient acceptance (bad taste,
odour, pain at injection site, etc.)
10. 10
Undesirable properties associated with drug molecules:
There are many undesirable properties associated with drug
molecules. These are:
1. Physical properties:
i) Poor aqueous solubility
ii) Low lipophilicity
iii) Chemical instability
2. Pharmacokinetic properties:
i) Poor distribution across biological membranes
ii) Good substrate for first-pass metabolism
iii) Rapid absorption/excretion when a long-term effect is
desired.
11. 11
Rationale for the use of prodrugs:
The rationale behind the use of prodrugs is generally to optimize
the “drug-like” properties (ADME- absorption, distribution,
metabolism, and excretion) because they can cause considerable
problems in subsequent drug development.
The prodrug approach is a very versatile strategy to increase the
utility of pharmacologically active compounds, because one can
optimize any of the ADME and toxicity properties.
In addition, the prodrug strategy has been used to increase the
selectivity of drugs for their intended target.
This can not only improve the efficacy of the drug but also
decrease systemic and/or unwanted tissue/organ-specific toxicity.
12. 12
A drug can only exert a desired pharmacological effect if
it reaches its site of action.
The three major phases involved in the drug receptor
interaction or biological bioavailability of drug include:
pharmaceutical phase
pharmacokinetic phase, and
pharmacodynamic phase.
Many barriers which limit drugs’ ability to reach a
desired target organ and the subsequent receptor site
are considered of pharmacokinetic origin.
Besides these, barriers of non-pharmacokinetic and
pharmacodynamic origin may also prevent a drug from
reaching the desired target.
14. 14
It includes –
pathological limitations such as toxicity, high incidence
of side effects and teratogenicity,
pharmaceutical limitations such as chemical instability
of product or formulation,
psychological limitations such as unpleasant taste, pain
at injection site and cosmetic damage to the patient,
and,
economic barriers.
Most of these limitations can be overcome by the
prodrug approach, but after overcoming the barriers,
the prodrug should rapidly convert into active moiety
after reaching the target site.
15. 15
The pharmaceutical phase can be considered as the
phase of development which involves the identification
of a new chemical entity with measured or proposed
therapeutic potential and its incorporation into a drug
delivery system.
The delivery system may be one of the traditional forms,
such as tablets, capsules, injections and creams/
ointments as well as the new drug delivery modes, such
as liposomes, implants, etc.
The pharmacokinetic phase can be considered as the
phase involving absorption, distribution, metabolism, and
excretion of the drug, and provide information regarding
the in vivo properties of a drug’s limitations, such as poor
absorption, too rapid elimination, and pre-systemic
metabolism.
16. 16
Targeted prodrug design:
Classical prodrug design often represents a non-specific
chemical approach to mask undesirable drug properties, such
as limited bioavailability, lack of site specificity, and chemical
instability.
On the other hand, targeted prodrug design represents a
new strategy for directed and efficient drug delivery.
Prodrugs can be designed to target specific enzymes or
carriers by considering enzyme-substrate specificity or
carrier-substrate specificity in order to overcome various
undesirable drug properties.
Steps in prodrug design:
Identification of the drug delivery problem
Identification of desired physicochemical properties
Selection of appropriate transport moiety.
17. 17
Objectives in prodrug research:
There are three basic, overlapping objectives in prodrug
research:
1. Pharmaceutical
2. Pharmacokinetic
3. Pharmacodynamic
1. Pharmaceutical Objectives:
The pharmaceutical objectives are - to improve solubility,
chemical stability, and organoleptic properties; to decrease
irritation and/or pain after local administration; and to reduce
problems related to the pharmaceutical technology of the
active agent.
18. 18
2. Pharmacokinetic Objectives:
Pharmacokinetic objective in the prodrug design serves as a
very important approach.
It facilitates in several ways: improvement in absorption (by
oral and non-oral routes), decreasing pre-systemic
metabolism to improve time profile, improvements due to
change in formulation, and increase in organ/tissue-selective
delivery of the active agent.
Foremost among these is a need to improve oral
bioavailability, be it by improving the oral absorption of the
drug and/or by decreasing its pre-systemic hepatic
metabolism.
Achieving improved oral absorption by a prodrug strategy is a
frequent rationale in marketed prodrugs.
19. 19
Other objectives are to improve absorption by parenteral
(non-enteral, e.g., dermal, ocular) routes and to increase the
duration of action of the drug by having a slow metabolic
release.
Zanamivir is a highly hydrophilic drug administered in
aerosol form.
The active agent is oseltamivir acid, which also shows very
high in vitro activity but low oral bioavailability due to its high
polarity.
To circumvent this problem, it is marketed as its ethyl ester
prodrug (oseltamivir), which undergoes rapid enzymatic
hydrolysis and produces high and sustained plasma levels of
the active agent.
20. 20
Pharmaceutical formulation is the most frequent method
used to achieve slow release and prolong the duration of
action of a given drug.
A conceptually different and an elegant approach to slow
metabolic release has been achieved with bambuterol, a
prodrug of the β2-adrenoreceptor agonist terbutaline.
Compared to terbutaline, bambuterol provides smooth and
sustained plasma levels of terbutaline, and a greater
symptomatic relief of asthma with a lower incidence of side
effects.
21. 21
One more pharmacokinetic objective of prodrugs is
organ/tissue selective delivery of a given drug.
A very significant example is that of capecitabine, a multistep,
orally active prodrug of 5-fluorouracil, an anticancer agent.
Capecitabine is well absorbed orally and undergoes three
activation steps resulting in high tumour concentrations of
the active drug.
It was first approved for the cotreatment of refractory
metastatic breast cancer.
Its therapeutic spectrum now includes metastatic colorectal
cancer.
Capecitabine thus affords an impressive gain in therapeutic
benefit compared to 5-fluorouracil due to its oral
bioavailability and a relatively selective activation in and
delivery to tumors.
22. 22
3. Pharmacodynamic Objectives:
Pharmacodynamic objectives aim to decrease the systemic
toxicity of a drug.
This might be the masking of a reactive agent to improve its
therapeutic index, or the in situ activation of a cytotoxic
agent.
The masking of a reactive agent to improve its therapeutic
index is suitably illustrated by the successful anti-aggregating
agent clopidogrel.
This compound, whose molecular mechanism of action was
poorly understood for years, is now known to be a prodrug.
In situ activation to a cytotoxic agent is part of the well-
known mechanism of action of the antibacterial and
antiparasitic nitroarenes such as metronidazole.
23. 23
The objectives in the prodrug design can be summarized as
indicated in the table below:
26. 26
Evaluation of prodrugs:
Once the drug/pharmacological agent is converted into a
prodrug, it is necessary to assess the new derivative (prodrug)
by using several parameters.
This is needed to check whether the synthesized prodrugs are
fulfilling the need and criteria of the aforesaid purpose or not.
This also gives direction to overcoming any unwanted
properties of the existing drug.
The evaluation of prodrug is also required to check the nature
of pro-moiety and its interference in physicochemical
property and activity as well.
This confirms the safety of the drugs and thus increases the
therapeutic effectiveness of the existing drug.
The various evaluation parameters are as follows.
27. 27
i) Physiochemical Parameters:
The physicochemical parameters affecting drug action include
solubility, lipophilicity, and partition coefficient.
This is also applicable to prodrugs.
ii) Pharmacokinetics Profile:
As derivatization and synthetic methods are necessary to
design a prodrug, of equal importance is its cleavage.
The action of the drug is associated with the drug molecule
only, and not with the prodrug.
Chemical and enzymatic hydrolysis is the guiding step in the
design of prodrugs.
Unless there is hydrolysis by one or other means the parent
drug will not be released.
Thus, the pharmacokinetics profile and release pattern are
vital for prodrugs.
28. 28
iii) Pharmacodynamics:
As every drug must have adequate therapeutic activity, so
too must every prodrug.
To appraise the prodrug completely, the final step is to check
the pharmacological activity.
Evaluation of activity is done in order to ensure the retention
or increase in the drug activity.
In some cases, such as mutual prodrugs, where both drug
and pro-moiety have activity, synergistic activity may result.
All these activities concern the pharmacodynamic profile of
the prodrug and are requirements in prodrug design.
29. 29
CLASSIFICATION OF PRODRUGS
The criteria for classifying prodrugs are:
1. Research-related criteria
2. Chemical criteria.
1. Research-Related Criteria
A historical discrimination can be made between deliberate
and accidental prodrugs.
The deliberately designed prodrugs are the result of chemical
derivatization of an active drug molecule.
Most prodrugs in clinical use are of this type, and they were
developed to improve the pharmaceutical and/or
pharmacokinetic properties of an active agent, be it a lead
candidate, a lead compound, a clinical candidate, a drug
candidate, or a marketed drug.
30. 30
A historically interesting example is that of hexamine.
Hexamine is an intentional prodrug of formaldehyde used
as a throat disinfectant for a long time.
Some prodrugs were accidental discoveries; the discovery of
the active agent they generate will contribute significantly to
understanding of their mechanism of action and may even
lead to the discovery of a new therapeutic class.
The most spectacular illustration of this was the discovery of
the antibacterial medicine, prontosil, which has been
converted to its active metabolite sulfanilamide.
31. 31
This has been widely used since its discovery in 1935.
The discovery of sulfa drugs was a milestone in medicinal
chemistry and all sulfonamides using today are as a result of
the discovery of prontosil.
32. 32
2. Chemical Criteria:
Another mode to classify the prodrugs is based on the
chemical point of view.
Based on chemical criteria prodrugs are sub-classified into
different classes.
1. Carrier-linked prodrugs
2. Mutual prodrugs
3. Bio-precursor prodrugs
4. Tripartite prodrugs
5. Polymeric prodrugs.
33. 33
1. Carrier-linked prodrugs:
This is a compound that contains an active drug linked to a
carrier group that can be removed enzymatically after
hydrolysis.
The bond to the carrier group must be liable enough to allow
the active drug to be released efficiently in vivo.
The carrier group must be nontoxic and biologically inactive
when detached from the drug.
Various adverse physicochemical properties of drug can be
tailored and side effects can be minimized by attaching a
nontoxic carrier group or pro-moiety to form a prodrug, from
which the parent drug is regenerated in vivo.
34. 34
A common example is dipivalyl ester of epinephrine,
which enhances corneal absorption and inhibits the
rapid metabolic destruction of epinephrine; reduces CV
side effects. – Dipivefrine- C19H29NO5
Dipivefrin is a prodrug of epinephrine formed by the
diesterification of epinephrine and pivalic acid.
35. 35
Depending upon the nature of the carrier used, the carrier-
linked prodrug may further be classified into:
1. Cascade-latentiated prodrugs:
A prodrug is further derivatized in a fashion such that only
enzymatic conversion to prodrug is possible before the latter
can cleave to release the active drug.
2. Double prodrugs, pro-prodrugs:
The double prodrug is a biologically inactive molecule which
is transformed in vivo in two steps (enzymatically or
chemically) to the active species.
As a novel example of improved entry of poorly delivered drugs into the brain by
means of nutrient conjugates, L-carnitine was conjugated to nipecotic acid and the
capacity to antagonize PTZ-induced convulsions of this novel entity was evaluated.
36. 36
3. Macromolecular prodrugs:
In these, macromolecules, such as polysaccharides, dextrans,
cyclodextrins, proteins, peptides, and polymers, are used as
carriers.
37. 37
4. Site-specific prodrugs:
In site-specific prodrugs, a carrier acts as a transporter of the
active drug to a specific targeted site.
recent prominent examples of marketed prodrugs is the
baloxavir marboxil, approved by the FDA in 2018 -
influenza (flu) antiviral drug
38. 38
2. Mutual Prodrug:
A mutual prodrug consists of two pharmacologically active
agents coupled together covalently so that each acts as a pro-
moiety for the other and vice versa.
The selected carrier may have the same biological action as
that of the parent drug and, thus might have a synergistic
action, or the carrier may have some additional biological
action that is lacking in the parent drug, thus ensuring
additional benefit.
39. 39
The carrier may also be a drug that might help to target the
parent drug to a specific site, organ, or cells, or may improve
the site-specificity of a drug.
The carrier drug may be useful to overcome some side effects
of the parent drug as well.
The mutual prodrug concept is useful when two synergistic
drugs need to be administered at the same site at the same
time.
40. 40
Mutual prodrugs are synthesized toward a pharmacological
objective of improving each drug’s efficacy, optimizing
delivery, and lowering toxicities.
Benorylate, which is a common example of this category, is a
ester linked prodrug of acetyl salicylic acid/Aspirin and
paracetamol.
A major advantage associated with this prodrug is in the
treatment of chronic inflammation at a decreased dose and
with a reduced risk of Gastric irritation.
42. 42
3. Bio-precursor Prodrug:
The bio-precursor does not contain a temporary linkage
between the active drug and carrier moiety, but is designed
from a molecular modification of an active principle itself.
It does not have any carrier or pro-moiety.
Numerous drugs are currently known which exert
pharmacological effects after their conversion into an active
metabolite.
Bio-precursor prodrugs are compounds that already contain
the embryo of the active species within their structure, i.e., it
contains latent functionality.
43. 43
A bio-precursor prodrug has a different structure that cannot
be converted into active drug by simple cleavage of a group
from the prodrug.
This active species is liberated by metabolic or chemical
transformation of prodrug.
The types of activation often involved are oxidative,
reductive, or phosphorylation; of these, oxidative activation
is common.
46. 46
4. Polymeric Prodrug:
The conjugation of a drug with a polymer is called a
polymeric prodrug.
These are biocompatible (non-toxic, non-antigenic, non-
teratogenic).
Polymeric prodrugs are designed to improve the use of drugs
for therapeutic applications.
Successful bioconjugation depends upon the chemical
structure, molecular weight, steric hindrance, and the
reactivity of the biomolecule as well as the polymer.
47. 47
The polymeric carrier can be either an inert or a
biodegradable polymer.
Selection of a suitable polymer and methodology to
conjugate the same with different bioactive components is a
critical step.
The drug can be pre-set directly or via a spacer group onto
the polymer backbone.
The proper selection of this spacer opens the possibility of
controlling the site and the rate of release of the active drug
from the conjugate by hydrolytic or enzymatic cleavage.
Eg: P-Phenylene diamine musterd is covalently attached to
polyamino polymer backbone of polyglutamic acid.
48. 48
Three major types of polymeric prodrugs are currently being
used:
i) Prodrugs that are broken down inside cells to form active
substance(s).
ii) Usually the combination of two or more substances (under
specific intracellular conditions, these substances react to form
an active drug).
iii) Targeted drug delivery systems, usually includes three
components: a targeting moiety, a carrier, and one or more
active components.
49. 49
Polymeric conjugates have several advantages over their low
molecular-weight precursors.
The main advantages include:
i) an increase in water solubility
ii) enhancement of drug bioavailability;
iii) protection of drug from deactivation and preservation of its
activity during circulation, and
iv) transport to the targeted organ or tissue
50. 50
IV. improvement in pharmacokinetics;
V. a reduction in antigenic activity;
VI. the ability to provide passive or active targeting of the drug
specifically to the site of its action and may include several
other active components that enhance the specific activity
of the main drug.
51. 51
5. Tripartite Prodrug:
Structures of most prodrugs are bipartite in nature, whereby
parent drug is attached directly to pro-moiety.
Bipartite prodrug comprised of one carrier is attached to the
drug.
However, in some cases bipartite prodrug may be unstable
due to the inherent nature of the drug-promoiety bond.
This can be overcome by designing a tripartite prodrug,
utilizing a spacer or connector group between the drug and
pro-moiety.
Tripartite prodrugs comprises of a carrier connected to a drug
through a linker.
52. 52
The spacer or connector group must be designed in such a
way that the initial activation is followed by spontaneous
cleavage of the remaining drug spacer bond under
physiological conditions to release parent drug.
Example: Pivampicillin.
Pivampicillin is a pivaloyl-oxymethyl ester tripartite prodrug
of the β-lactam antibiotic, ampicillin.
The prodrug uses a –CH2 linker to link ampicillin and the
pivalic acid carrier.
Pivampicillin is thought to have greater bioavailability than
ampicillin because the ester group grants the compound
greater lipophilicity.
54. 54
APPLICATIONS OF PRODRUG DESIGNING:
1. Masking Taste and Odor:
Taste and odor present problems, particularly with pediatric
patients.
One of the reasons for poor patient compliance, particularly
in the case of children, is bitterness, acidity, or causticity of
the drug.
Masking of undesirable taste and odor is needed to
overcome poor patient compliance.
Two approaches can be utilized to overcome the bad taste of
drugs.
The first is reduction of drug solubility in saliva and the other
is to lower the affinity of drug towards taste receptors.
55. 55
The undesirable taste arises due to adequate solubility and
interaction of drug with taste receptors, which can be solved
by lowering the solubility of drug or prodrug in saliva.
Chloramphenicol, an extremely bitter drug, has been
derivatized to chloramphenicol palmitate, a sparingly soluble
ester.
It possesses low aqueous solubility, which makes it tasteless
and later undergoes in vivo hydrolysis to active
chloramphenicol by the action of pancreatic lipase.
56. 56
Odor is another aesthetic concern for some drugs that are
often volatile liquid or solids with significant vapor pressure
that makes them difficult to formulate.
A classic example is the volatile mercaptans used as
tuberculostatic agents for the treatment of leprosy.
Ethyl mercaptan has a boiling point of 25 0C and a strong
disagreeable odor.
On the other hand, diethyl dithio isophthalate, a prodrug of
ethyl mercaptan, has a higher boiling point and is relatively
odorless.
57. 57
2. Minimizing Pain at Injection Site:
Pain at the injection site, especially after intramuscular
injection, can be caused by transfer of the drug into
surrounding cells and damage to the subcutaneous tissue.
It is caused by intramuscular injection mainly due to the
weakly acidic nature or poor aqueous solubility of drugs.
Such pain is usually associated with hemorrhage, edema,
inflammation, and tissue necrosis.
58. 58
Some of these problems relate to dose vehicle composition or
pH.
For example, intramuscular injection of antibiotics like
clindamycin was found to be painful due to poor aqueous
solubility and could be overcome by making phosphate ester
prodrugs respectively and maintaining the formulations at pH
12.
The 50-fold greater solubility of the 2-phosphate ester
prodrug of clindamycin results in little local pain or irritation
on injection.
The prodrug possesses little or no intrinsic antibacterial
activity but is rapidly converted to the parent drug by the
action of phosphatase in the body.
59. 59
3. Alteration of Drug Solubility:
Poor aqueous solubility can often prevent a drug from
achieving its full therapeutic potential.
For orally administered drugs, poor solubility in the
intestinal contents would result in poor absorption.
The prodrug approach can be used to increase or decrease
the solubility of a drug, depending on its ultimate use.
Many prodrugs designed to increase water solubility involve
the addition of an ionizable pro-moiety to the parent
molecule.
Because charged molecules have greater difficulty crossing
biological membranes, one must balance increased water
solubility with the potential for decreased permeability.
Prodrugs are also designed to improve aqueous solubility for
parenteral administration.
60. 60
For example, chloramphenicol succinate and
chloramphenicol palmitate, ester prodrugs of
chloramphenicol, have enhanced and reduced aqueous
solubility, respectively.
On the basis of altered solubility, chloramphenicol sodium
succinate prodrug is suitable for parenteral administration.
The prodrug approach is also useful for better GI absorption.
It was observed that sulindac, a prodrug of sulindac sulfide,
being more water-soluble with sufficient lipophilicity, makes
this drug suitable for oral administration.
61. 61
4. Enhancement of Chemical Stability:
Chemical stability is an extremely necessary parameter for
every therapeutic agent to elicit its pharmacological activity
for a longer duration.
Many drugs are unstable and may either breakdown on
prolonged storage or degrade rapidly on administration.
Several drugs may decompose in the GI tract when used
orally.
62. 62
Although chemical instability can be solved to a greater
extent by appropriate formulations, particularly enteric
coatings, its failure necessitates the use of prodrug approach.
The prodrug approach is based on the modification of the
functional group responsible for the instability or by changing
the physical properties of the drug, resulting in a reduction in
the contact between the drug and the media in which it is
unstable.
The prodrug approach can be successfully used to overcome
the various pharmacokinetic barriers, thereby improving the
therapeutic value of parent drug.
63. 63
The principal barriers identified in the pharmacokinetic phase
are:
i) Incomplete absorption of the drug from the delivery system
or across biological barriers such as the GI mucosal cells and
the blood-brain barrier.
ii) Incomplete systemic delivery of an agent due to pre-systemic
metabolism in the GI lumen mucosal cells and liver.
iii) Toxicity problems associated with local irritation or
distribution into tissue other than the desired target organ.
iv) Poor site-specificity of the drug.
64. 64
5. Prodrugs to Overcome Absorption Problems:
Poor absorption of drug may be due to physicochemical
properties of the drug itself.
Bioavailability after oral dosing of various water insoluble
agents is often dissolution rate-limited, whereas the
absorption of highly polar agents is often limited by their
transport across the GI cell membrane.
65. 65
Since most drugs are absorbed by passive diffusion, a degree
of lipophilicity is necessary for efficient absorption through
the GI barrier.
For highly polar compounds, the administration of less polar
and more lipophilic prodrug promotes GI absorption.
Also, many drugs are poorly absorbed into the central
nervous system, eye, or through the skin due to their highly
polar nature and a prodrug approach helps in overcoming
these barriers.
66. 66
5.1. Enhancement of Oral Absorption:
Various therapeutic agents, such as water-soluble vitamins,
structural analog of natural purine and pyrimidine
nucleoside, dopamine, antibiotics, such as ampicillin and
carbenicillin, phenytoin, and cardiac glycoside, such as
gitoxin, suffer from poor GI absorption.
The prime cause of the poor absorption of these agents is
their highly polar nature, poor lipophilicity, and/or
metabolism during the absorption process.
This problem could be manipulated successfully by using the
prodrug approach.
The absorption of water-soluble vitamin was enhanced by
derivatization of thiolate ions to form lipid-soluble prodrugs.
Dopamine was made useful by making its precursor L-dopa.
67. 67
Although L-dopa is highly polar, it is actively transported
through a specific L-amino acid active transport mechanism
and regenerates dopamine by decarboxylation.
This approach has been adopted with various penicillin
antibiotics.
Ampicillin is zwitterionic in the pH range of the GI tract and is
only about 20-60% absorbed following oral dosing.
Esterification of the carboxyl group of ampicillin to form the
prodrugs pivampicillin, becampicillin, or talampicillin alters
the polarity of the molecule and successfully improves oral
bioavailability.
68. 68
5.2. Enhancement of Ophthalmic Absorption:
The usefulness of epinephrine as an adrenergic agent in the
treatment of glaucoma is limited due to its highly polar
nature.
The dipivalyl derivative of epinephrine, formed by the
acylation of phenolic hydroxyl groups, showed enhanced
therapeutic effectiveness.
Lipid solubility of dipivalyl derivatives is far superior to its
parent compound, which facilitates its transport through a
lipoidal barrier during corneal absorption.
5.3. Enhancement of Percutaneous Absorption:
Mefenide and corticosteroid are used in the treatment of
inflammatory, burn therapy, allergic, and pruritic conditions,
but have limited application due to poor percutaneous
absorption.
69. 69
It was observed that mefenide hydrochloride and mefenide
acetate salt showed better responses than the parent drug.
The problem of poor percutaneous absorption of
corticosteroid was overcome by making various ester
prodrugs.
6. Prevention of Presystemic Metabolism:
Many drugs are efficiently absorbed from the GI tract but
undergo presystemic first-pass metabolism or chemical
inactivation before reaching the systemic circulation.
Phenolic moiety, oxidative N- and O-dealkylation, ester
cleavage, and peptide degradation are responsible for the
presystemic metabolism of various drugs.
In fact, two types of drug fall into this category.
The first are drugs rapidly degraded by the acid condition of
the stomach;
70. 70
Second, drugs that degrade due to enzymes present in the GI
mucosa and liver.
Enzymatic degradation is perhaps of greater significance than
chemical degradation.
Following oral administration, a drug must pass through two
metabolizing organs, i.e., liver and GI mucosa, before
reaching the general circulation.
Rapid metabolism of drugs in these organs is termed the
first-pass effect.
The first-pass metabolism of a drug can be prevented if the
functional group susceptible to metabolism is protected
temporarily by derivatization.
Alternatively, manipulation of the drug to alter its
physicochemical properties may also alter the drug-enzyme
complex formation.
71. 71
The prodrug approach was successfully used to overcome the
problem of considerable metabolism of steroidal drugs,
propranolol, dopamine, morphine, and catecholamines, by
making acetylated derivates of various steroids, 17α, 21-
acetonides of various corticosteroids, hemisuccinate ester of
propranolol, L-dopa prodrug in the case of dopamine,
diacetyl prodrug of morphine and ibuterol and bitoterol
prodrugs of terbutaline and N-(t-butyl arternol), respectively.
All the prodrugs, besides enhancing absorption and
bioavailability, protect the therapeutic agent from
metabolism.
7. Longer Duration of Action:
Drugs with a short half-life require frequent dosing with
conventional dosage forms to maintain adequate plasma
concentration of the particular drug.
72. 72
Frequent dosing for drugs rapidly cleared from the body
results in a sharp peak and valley effect.
In plasma level time profile, and consequently patient
compliance, is often poor.
The peak and valley effect can be minimized by drug delivery
at a controlled and predictable rate, such as zero order
delivery.
Prolongation of duration of action of a drug can be
accomplished by the prodrug approach and can take two
forms.
First, the input of drug in to the body can be controlled by a
prodrug/drug delivery formulation complex, which by design
releases drug at a controlled rate at the absorption site,
followed by conversion to drug prior to or just after
absorption.
73. 73
Second, a prodrug can be designed wherein the conversion to
the parent drug becomes the release rate-limiting factor in
the systemic milieu.
The present approach is most useful in the case of
neuroleptic drugs to avoid large fluctuations in plasma levels.
This could be successfully achieved by administering
hepatanoate and decanoate esters of fluphenazine in sterile
sesame oil.
Similarly, the problem of testosterone is overcome by
administering 17-propionate ester, 17-phenylacetylate, and
17-cypionate ester of testosterone in oil vehicle.
74. 74
8. To Diminish Local and Systemic Toxicity of Drugs/
Reduction of GI Irritability:
One of the desired properties in drug design and targeting is
to have therapeutic activity without toxicity.
It seems very difficult unless site specific delivery of drug is
achieved.
Some examples exist where a prodrug approach has resulted
in a reduction in toxicity.
Various NSAIDs, such as salicylic acid and indomethacin,
severely damage the GI mucosa due to the presence of a free
carboxylic group.
Aspirin has already been mentioned as a less corrosive
prodrug of acetylsalicylic acid.
75. 75
9. Site-Specific Drug Delivery:
Site-specific drug delivery attempts to obtain very precise
and directs effect at the site of action, without subjecting the
rest of the body to a significant level of active agent.
Certain limitations to this approach have been highlighted as
follows:
i) Adequate accessibility of prodrug to the target site
ii) Distribution, relative activity, and specificity of reconverting
enzymes
iii) Parent drug leakage from the target site
iv) Access of parent drug to the target site within the target
organ
v) Pharmacokinetics of the prodrug and drug molecules
vi) Toxicity of prodrug moiety and its metabolites
vii) Lack of information on the selective location of potential
reconverting enzymes.
76. 76
10. Prodrug for Slow and Prolonged Release (Sustained
Drug Action):
A common strategy in the design of slow-release prodrug is
to make long-chain aliphatic esters, because these esters
hydrolyze slowly, and to inject them intramuscularly.
Fluphenazine has a shorter duration of action (6-8 h), but
prodrug fluphenazine decanoate has a duration of activity of
about a month.
77. 77
CASE STUDIES IN PRODRUG DESIGN:
1. Ampiroxicam: An Anti-Inflammatory Prodrug Agent:
Piroxicam is a NSAID that can cause serious gastrointestinal
bleeding, perforation, and ulceration.
Anthony Marfat (Pfizer Inc., USA) discussed the use of the
prodrug concept that led to the discovery of ampiroxicam as
an anti-inflammatory agent that reduces the local or topical
component of gastric irritation.
This has been done by masking the enolic hydroxyl group of
piroxicam.
Over 225 derivatives of piroxicam were made and screened
for cyclooxygenase inhibitory activity, solution bioavailability
in rats, and solid-state stability and absorption in dogs.
Ampiroxicam was found to meet all decision criteria and was
developed further.
79. 79
2. Olmesartan Medoxomil: A Prodrug of Olmesartan:
Olmesartan is a durable, specific, and competitive
nonpeptide angiotensin II receptor antagonist used to treat
hypertension.
To achieve better oral bioavailability its ester prodrug,
olmesartan medoxomil, was developed.
Olmesartan medoxomil is suitable for once-daily oral
administration and is available in tablet form.
80. 80
3. Valacyclovir: A Prodrug of Acyclovir:
Valacyclovir is an L-valyl ester prodrug of acyclovir that is
used for the treatment of herpes, varicella zoster, and
cytomegaloviruses.
Valacyclovir was developed to increase the oral absorption
and plasma levels of acyclovir.
Increased plasma concentrations of acyclovir are important in
maintaining antiviral activity, especially in
immunocompromised patients and in the treatment of less
sensitive viruses.
Suboptimal exposures can lead to more resistant viral strains.
To achieve high enough exposures, acyclovir must be dosed
intravenously or in multiple high doses
